docs: def asset_url_for(self, asset):
          """
          Lookup the hashed asset path of a file name unless it starts with
          something that resembles a web address, then take it as is.
  
          :param asset: A logical path to an asset
          :type asset: str
          :return: Asset path or None if not found
          """
          if '//' in asset:
              return asset
  
          if asset not in self.assets:
              return None
  
          return '{0}{1}'.format(self.assets_url, self.assets[asset])